Abstract
Wettability, a fundamental physicochemical property of functional materials that depends on their surface structure, has seen recent advances in understanding that have led to the development of superhydrophobic surfaces with applications in self-cleaning materials and antifogging coatings. Given the presence of a free energy barrier between the Cassie and Wenzel states, calculating this barrier provides a valuable means to assess surface hydrophobicity. We hypothesize that surface characteristics affect the free energy barrier between wetting states and that the determination of this barrier can be used to predict the hydrophobicity. Focused on models of a butterfly wing (striped surface) and a mosquito eye (pillared surface), this study employed many-body dissipative particle dynamics (mDPD) simulations to explore the relationship between the free energy barrier and surface roughness (height and spacing). For small-radius droplets, the height of the surface pattern predominantly influences the hydrophobicity. Conversely, for large-radius droplets, the spacing of the surface pattern is the dominant factor. Considering that the contact angle indicates static hydrophobicity, the free energy barrier is the indicator of dynamic hydrophobicity, which shows how well the surface can maintain its hydrophobicity under the impact of a droplet. This study underscores the advantages of the free energy barrier method in understanding dynamic hydrophobicity, which can be applied in evaluating high kinetic energy droplets onto surfaces, for example, automobiles, trains, and aircraft.